1. Field of the Invention
The present invention relates to an image pickup device such as a video camera.
2. Related Background Art
Recently, video apparatuses represented by the video camera have been drastically advanced, and a variety of automatic functions and the improvement in the operativity have been achieved. For example, automatic features such as the provision of a zoom lens, the automatic focus adjustment and the automatic exposure control are requisite. The automatic exposure control is an important factor in determining the quality of picked-up image, and has the great importance in all photographing environments because stable and excellent automatic exposure control is always required.
FIG. 1 is a block diagram showing the basic configuration of an exposure control system in a typical video camera. 101 is a taking lens optical system, 102 is an iris for adjusting the amount of incident light, 103 is an image pickup element such as a CCD for converting photoelectrically an image formed on an image pickup plane by the taking lens optical system, with the amount of light adjusted by the iris, into an image pickup signal, 104 is a camera signal processing circuit for applying a predetermined signal processing to the image pickup signal output from the image pickup element to obtain a normalized video signal, 105 is a video signal output terminal, 106 is a motor for varying the amount of opening by driving the iris, 107 is an iris driving circuit for driving and controlling the motor 106, 108 is a CCD driving circuit for setting a desired shutter speed by variably controlling the accumulation time (exposure time) as well as the accumulation, reading and reset timing of the image pickup element 103, 109 is an automatic exposure control circuit (AE circuit) for optimally controlling the exposure by evaluating the exposure state based on the level of a luminance signal out-put from the camera signal processing circuit and controlling the iris driving circuit 107 and the CCD driving circuit 108, and 110 is a switch panel for accepting the input of key operation.
For the exposure control, the AE circuit 109 is provided with a control system for controlling the iris driving circuit 107 by integrating the luminance signal output from the camera signal processing circuit 104 so that the level of integration may fall within a predetermined range, whereby a closed loop for the iris control is constituted to control the drive current to be output to the iris motor to vary the amount of opening in the iris, and the proper exposure state is obtained by controlling the CCD driving circuit 108 in accordance with the key operation on the switch panel 110, thereby changing the driving pulse to vary the accumulation time of image pickup element 103, and thus the exposure time or shutter speed.
This accumulation time control relies on a so-called electronic shutter, and allows for the selection of the light accumulation time at a plurality of steps from about 1/100 to 1/10000 second, in addition to the normal exposure time of 1/60 second per picture, in NTSC method, for example.
When a high speed shutter is used, the system thus constituted is placed in a so-called shutter speed priority mode because of the action of an automatic exposure control mode for controlling a stop mechanism (iris) of the image pickup optical system based on a set exposure time or shutter speed which is arbitrarily selected. FIG. 2 shows the shutter speed priority mode, in which the shutter speed in the transversal axis is selected, and then the iris value in the longitudinal axis is varied with the shutter speed fixed.
However, as with the above-described video camera, in the shutter speed priority mode with the iris control based on the luminance level of an image pickup signal, the exposure control is not always appropriately made in various photographing environments and photographing situations, and frequently resulted in inappropriate exposure control.
In particular, in a camera for taking a still image instantaneously such as a silver salt camera, the appropriate exposure control at an instant of photographing is requisite, but as with a video camera, when photographing of a motion picture is made for a long time, the optimal exposure control is always necessary and must be made stably in the momentarily changing, photographing situation and image pickup environment, whereby the realization of an exposure control feature for the video camera meeting such conditions is eagerly desired.
Thus, in order to make the optimal photographing operation corresponding to all the photographing environments at all times, the photographing modes corresponding to various photographing situations are prepared and selected depending on a situation of photographing.
However, in the case where a plurality of photographing modes are switchable, the characteristics and set values of control parameters may be abruptly changed when the mode is switched, whereby there is a risk that unnatural change may occur, and the quality of image may be degraded, in particular, in a video camera for taking a motion picture.